HP-45

The HP-45 was HP's first scientific calculator to have a shift key. This
was such a novel feature at the time that HP credited the key with nearly
doubling the computational power of the calculator. (Advertisement July '73
Scientific American.)

The HP-45 added the ability to do trig functions in Radians or Grads, polar
to rectangular conversions, decimal degrees to/from degrees minutes seconds,
basic statistics %, % change, n! etc. It was also the first pocket calculator,
to have 9 storage registers in addition to the 4 level stack. "(Let your
imagination picture the calculating horsepower of this feature for register
arithmetic, conversions, continued products, payrolls...)" (Advertisement
July '73 Scientific American.)

The HP-45 also added 10^x and replaced x^y with y^x. It also added the now
ubiquitous Last X key and x^2. The latter was placed on an unshifted key
where it makes sense. (So it actually takes one less keystroke to do x^2
than Enter, multiply.)

Another feature added on the HP-45, was the formatted display. It added both
fixed and scientific displays with a choice of the number of digits displayed.
The HP-35 had only what is now commonly called "standard
mode". In fixed mode the HP-45 would display sufficiently small numbers as
0. Starting with the 20 series, HPs automatically switched to scientific
display when this would happen.

Conversion Factors

It also added conversion factors for a few units like in/cm. When
you pressed one of these conversion factor keys, the conversion factor was
entered into the x register and then the user could use it to multiply or
divide as needed.

How to Clear The HP-45

Unlike the HP-35, this calculator had no "clear the
entire machine" button. However, its back plate points out that you can clear
everything by turning the machine off and then on.

Quotes from the Introduction

"Little is understood about the methods used by calculating prodigies to
perform their awesome feats. The method used by 10-year-old Truman Henry
Safford, in 1846 to calculate 365365365365365365^2 (as described by the Rev.
H. W. Adams) shows that difficult problems are difficult even for
prodigies--"...he flew around the room like a top, pulled his pantaloons
over the tops of his boots, bit his hands, rolled his eyes in their sockets,
sometimes smiling and talking and then seeming to be in agony, until, in
not more than a minute said he, 133,491,850,208,566,925,016,658,299,941,583,225!"

"Although your HP-45 might not be as much fun to watch, it makes calculating
faster and less arduous, because the operational stack and the reverse "Polish"
notation used by the HP-45 provide the most efficient way known to computer
science for evaluating mathematical expressions."

And from a bit later in the introduction:

"Incidentally, no calculator available today (including ours) can handle
the problem given to our child prodigy. Isn't it comforting to know that
people can still do things machines can't?"

A Timer

The HP-45 had code to implement a timer. This was never exposed as a user
feature because the lack of a quartz crystal in the HP-45 made the timer
fairly inaccurate. The timer could be invoked by pressing RCL and then pressing
CHS 7 8 all at the same time. Once in timer mode, CHS toggled it between
running and stopped. Pressing 1-9 stored the current time in that register
when the timer was running or recalled the stored time when the timer was
stopped. Pressing the decimal point key, brought the calculator back to normal
mode with the time still in the display. (Pressing Enter also resumed normal
mode but cleared the display.) It's possible to add
a crystal to the HP-45.

Versions

The HP-45 can be found with "On" and "Off" molded or painted on. Like the
HP-80, early versions had a metal strip above the power switch to avoid wear
on the silver trim.